Aqueous polytetrafluoroethylene-based fluororesin dispersions, when applied by such a method as coating or impregnation, can form films excellent in such characteristics as chemical stability nonstickiness and weathering resistance and therefore have been widely used in such fields of application as cooking utensils, pipe linings and impregnated glass cloths.
In such fields of application, it is preferable that the aqueous fluororesin dispersions should be high in fluororesin concentration; therefore, they are generally used after concentration following polymerization of a fluoromonomer or fluoromonomers in an aqueous medium in the presence of a fluorinated surfactant. However, in recent years, it has been desired that the content of the fluorinated surfactant in the products should be reduced.
The so-far proposed method of removing a fluorinated surfactant from an aqueous fluororesin dispersion comprises repeating a plurality of times the procedure comprising warming the dispersion in the presence of a nonionic surfactant for causing phase separation, separating the supernatant and recovering the lower phase (Patent Document 1), bringing the dispersion into contact with an anion exchange resin in the presence of a nonionic surfactant to thereby remove the fluorinated surfactant (Patent Document 2) or removing the fluorinated surfactant using an ultrafiltration membrane (Patent Document 3), for instance.
However, the dispersions deprived of the fluorinated surfactant by the methods mentioned above are insufficient in mechanical stability and, therefore, when exposed to mechanical shear on the occasion of transportation or processing thereof, they readily allow the formation of polymer coagula. Since they are also insufficient in storage stability, another problem arises: when they are stored standing, sludge is formed on the container bottom with the lapse of time.
Further, a PTFE dispersion in which each particle comprises a high-molecular-weight core and low-molecular-weight shell has been proposed as a method of improving the stability of the dispersion (Patent Document 4). However, any PTFE dispersion having a core-shell structure and containing substantially no fluorinated surfactant is unknown in the art.    Patent Document 1: International Publication WO 2004/050719    Patent Document 2: Japanese Kohyo (Laid open under PCT) Publication 2002-532583    Patent Document 3: Japanese Kokai Publication S55-120630    Patent Document 4: U.S. Pat. No. 6,841,594